1. Field of the Invention
The present invention relates in general to a printing apparatus wherein a printing operation is effected by a print head which is movable relative to recording medium while the print head and the medium are held in pressed contact with each other. More particularly, the invention is concerned with an improved mechanism for moving the print head of such a printing apparatus to its printing and release positions. The present invention further relates to an improved mechanism for pressing a thermal ink ribbon against the recording medium.
2. Discussion of the Prior Art
A thermal printer is a typical example of the printing apparatus indicated above. One type of the thermal printer uses a thermal ink ribbon having a thermally fusible ink layer which is pressed by the print head against the surface of the recording medium, so that local portions of the ink layer heated by the print head are transferred to the recording medium. There is known another type of thermal printer which employs a heat-sensitive recording medium whose local portions heated by the print head are blackened or otherwise colored. In any types of thermal printer as described above, the print head is movable to its printing position in which the print head is in pressed contact with the recording medium, and its release position in which the print head is spaced away from the medium. Therefore, the thermal printer should include a print-head support member for movably supporting the print head, and an actuator mechanism for moving the print head to its printing and release positions.
In the known thermal printer, it is common that the print-head support member is biased by a spring so that the print head is biased toward its recording medium. Namely, the print head is normally placed in its printing position. In this arrangement, a cam or other actuator mechanism used to move the print head away from the recording medium to its release position should overcome the biasing force of the spring, which increases as the print head is moved toward the release position. The biasing spring is pre-loaded so as to provide a desired pressing force which acts on the print head held in its printing position. Since the movement of the print head toward its release position results in an increase in the amount of elastic deformation of the biasing spring, the load which is applied to the actuator mechanism must be greater than the biasing force acting on the print head in the printing position, and therefore requires a comparatively large drive source.
In the light of the above drawback, an improved thermal printer is proposed in Japanese patent application published on May 4, 1985 under Publication No. 60-17712 (for opposition purpose). In this printer, the print head is normally placed in its release position by a first biasing member, and a solenoid-operated print-head actuator is connected to the print head via a second biasing member. When a printing operation is initiated, the drive force of the solenoid is imparted via the second biasing member to the print head, to move the print head from the release position to the printing position, while overcoming the biasing force of the first biasing member. After the print head is moved to the printing position, the second biasing member is elastically deformed by the solenoid, whereby the print head is pressed against the recording medium. In the present arrangement, since the biasing force to hold the print head in the release position may be very small, the required drive force produced by the solenoid is substantially equal to the desired pressing force which acts on the print head, and therefore the required capacity of the solenoid is relatively small.
Since the solenoid is used as a drive source to move the print head to its printing position, the print head abuts on the platen of the printer at a comparatively high speed, causing considerably high magnitudes of impact of the print head against the platen and consequent operating noises. Therefore, the print-head actuator using such a solenoid is not desirable from the standpoint of durability of the print head and the actuator mechanism, and damages to the recording medium. In this connection, it is noted that the solenoid is difficult to control its operating speed. Namely, it is difficult to control the moving speed of the print head to its printing position. Even if the print head moving speed could be made lower by some means or other to solve the above inconveniences, the instant arrangement still suffers from a relatively long total operating time required to move the print head to the printing position and press the print head against the recording medium (platen).
Similar problems are encountered with respect to a ribbon-presser mechanism for pressing a ribbon-presser member used to press the ink ribbon against the recording medium, where the thermal printer has a function of erasing printed characters by means of the ink ribbon. This type of thermal printer is disclosed in Japanese patent application laid open on Feb. 24, 1983 under Publication No. 58-31787 which corresponds to U.S. Pat. No. 4,396,308.
Described more specifically, the thermal printer having such an erasing function has a ribbon-presser member in the form of a roller, for example, which is located at a position spaced away from the print head by a suitable distance along the line of printing. The ribbon-presser member is supported movably between its non-operated and operated positions. In the operated position, the ribbon-presser member is held in pressed contact with the ink ribbon. The ink ribbon has an ink layer consisting of an inking material which produces a sufficient adhesive force at temperatures between the room temperature and the printing temperature of the ribbon. A printing operation is effected while the ink ribbon pressed by only the print head against the recording medium, as described above. When printed characters are erased, the ink ribbon is pressed against the recording medium, by both the print head and the ribbon-presser member. The print head is operate according to the same printing data as used for the already printed characters, whereby the printed characters are overstruck by the print head via the ink ribbon. The used portion of the ink ribbon is separated from the recording medium after the used portion is kept in pressed contact with the medium for a time corresponding to the distance between the print head and the ribbon-presser member. As a result, the locally heated portion of the ink ribbon are cooled down from the printing temperature to a temperature at which the inking material demonstrates a considerably high adhesive strength, which causes the previously transferred inking material (forming the printed characters) to adhere back to the ink ribbon. Thus, the inking material of the printed characters may be removed from the recording medium by the ink ribbon.
Thus, the thermal printer having the ribbon-presser member requires a ribbon-presser mechanism for moving the ribbon-presser member between its non-operated and operated positions, and therefore suffers from the same problems as experienced by the print-head presser mechanism.